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Mosca et al.: Commutability of controls

Abbot IMx

0.44

0.75

0.35

0.83

0.08

0.76

Boehringer Tina-quant

0.18

0.50

0.97

0.53

0.61

0.53

Roche Unimate

1.03

0.53

0.75

0.50

0.61

0.47

Menarini H8140

1.03

0.24

0.01

0.22

0.03

0.22

Merck–Hitachi L-9100

2.03

0.36

0.26

0.36

0.10

0.36

Tosoh Alc 2.2

0.83

0.27

0.44

0.28

0.09

0.27

Kontron s400

2.00

0.23

0.05

0.24

0.11

0.23

HbA1c

Table 3. Intermethod differences of Hb A1c measurements on blood (Bio-Rad Variant was assumed as x-method).

y-method

Raw data

Gly corrected

Sam corrected

Bias [(x-method)

( y-method)], A1c, % of total Hb

(mean

SD)

Standardization Program (IGSP). It is actually impossible to anticipate if the NGSP and the IGSP will converge to the same reference and target values and which of the two Standardization Programs will involve the greater num- ber of manufacturers. As a time provision, in our personal experience the NGSP trial required approximately 6 months before getting the certification (www.missouri. edu/ diabetes/ngsp/) and we estimate that the IGSP may take probably the same amount of time. It is becom- ing common opinion that the practical benefit return in terms of glyHb standardization will not come about before a couple of years from now [22].

Fig. 2. Cumulative distributions of the differences between various methods (y-methods) and Bio-Rad Variant (x-method) with regard to the Hb A1c measurement in blood samples.

The units plotted on the x-axis are Hb A1c concentrations, expressed as a percentage of total Hb. (A) Original data; (B) data corrected after calibration with Menarini controls; (C) data corrected after calibration with home-prepared control materials. E, Abbott IMx; F, Boehringer Tina-quant; ƒ, Roche Unimate HbA1c;

, Menarini H8140; f, Merck–Hitachi L-9100; Œ, Tosoh A1c 2.2; , Kontron s400. Dashed lines are drawn at centiles 5, 50, and 95; a continuous vertical line is drawn at 0 bias.

account differences in sample residuals among the vari- ous comparisons, it is noteworthy that three methods (Roche, Menarini, and Kontron) had slopes significantly

different (P

0.024) from a value of 1.0. This is another

confirmation of the need for glyHb standardization.

To reduce bias among methods, different approaches to glyHb standardization have been launched, these being essentially the one promoted first by the National Glyco- hemoglobin Standardization Program (NGSP) and, later on, that diffused by the International Glycohemoglobin

While awaiting for international glyHb standardiza- tion at the manufacturers’ level, in some countries na- tional standardization activities have been started with commercially available [23] or home-prepared control materials ([10, 11, 24]). With regard to the commercial controls, some manufacturers report limitations in the intended use of their controls, and some do not. In some cases (Sweden, Germany) serious difficulties to get good control materials were reported, especially because of considerable matrix effects found for some methods (J.O. Jeppson and R. Kruse, personal communications). For these reasons, we decided to perform a homogeneous and comprehensive commutability study on most of the cur- rently available control materials, by comparing the re- sults with those previously obtained in an Italian stan- dardization study with home-prepared controls ([11]). For technical reasons it has not been possible to include in our study all the techniques and materials used for glyHb analysis. For instance, we regret not to have analyzed methods such as the Bayer DCA 2000 and the Primus affinity HPLC, both very popular in some countries. However, since new methods and materials are continu- osly coming to the light, the DCA 2000 and the Primus methods, together with other methods and materials, could be the object of future investigations on calibration and commutability.

Therefore, the intrinsic characteristics of various hemo- lysates proposed as controls for glyHb determinations were investigated. Since commutability depends not only on the methods tested, but also on the nature and source

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